mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 11

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

Do not run high frequency conductors near, and particularly underneath, the crystal, the feedback resistor
or the series resistor. In Figure 6, only a ground trace runs underneath these components. Also note that,
in Figure 6, the ground trace is tied to the ground pin nearest to the oscillator pins. This helps prevent large
loop currents in the vicinity of the crystal. It is also very important to tie the ground pin to the most solid
ground in the system. The trace that connects the oscillator and the ground plane should not connect to any
other circuit element as the injection of current into this trace tends to make the oscillator unstable.
2.5.2.4 XFC and V
Noise on the XFC, V
the V
digital logic coupling to either XFC or V
directly to the V
V
A V
capacitor from V
amining the frequency of the V
supply noise. An inductive filter would replace the resistor with an inductor.
The low-pass filter requires an external low-leakage capacitor, typically 0.1 F with an insulation resistance
specification as high as practical. The main criterion is that the capacitor be low-leakage because leakage
affects frequency stability and accuracy. Do not use a tantalum capacitor. Although the SIM Reference Man-
ual (SIMRM/AD) recommends an insulation resistance of 30,000 M , this value may not be necessary in
all applications. For most consumer (room temperature) applications, polystyrene capacitors are recom-
mended. See Figure 7 for a recommended circuit.
2.5.2.5 Evaluating Oscillator Performance
Once an entire oscillator circuit is built, it is very important to evaluate circuit characteristics. Of particular
interest is how the oscillator starts. If the oscillator starts in a metastable state that persists for several hun-
dred milliseconds, it is quite possible that this state will persist until the MCU releases reset and tries to start
fetching instructions. When this happens, the PLL may well be operating at a frequency far greater than the
MC68331/332
M68331/332TUT/D
DDSYN
DDSYN
DDSYN
may require an inductive or resistive filter to control supply noise.
resistive filter would consist of a 100 to 500
bypass capacitors should be kept as close to the XFC and V
SSI
Some published errata sheets and user’s manuals recommend a filter circuit that
includes an 18 k
investigation has shown that, when this circuit is used, if there is leakage (about 50
k ) between the XFC pin and the power supply, the MCU may not come out of re-
set because the internal VCO lock detect circuitry does not operate properly. Use
the circuit shown in Figure 7 instead.
DDSYN
*
MAINTAIN LOW LEAKAGE ON THE XFC NODE.
DDSYN
ground plane. If possible, route V
DDSYN
V
SSI
to V
C3
C4
0.01 F
0.1 F
, and V
Figure 7 Conditioning the XFC and V
SSI
Freescale Semiconductor, Inc.
For More Information On This Product,
. The proper values for the resistor and capacitor can be determined by ex-
DDSYN
SSI
resistor for a high stability operating environment.Subsequent
pins causes frequency shifts in CLKOUT. The XFC filter capacitor and
noise. The RC time constant needs to be large enough to filter the
DDSYN
Go to: www.freescale.com
C1
0.1 F
. The ground for the V
NOTE
XFC
V DDSYN
DDSYN
resistor from V
*
and V
DDSYN
DDSYN
SSI
as separate supply runs or planes.
DD
bypass capacitors should be tied
Pins
DDSYN
to V
DDSYN
332TUT XFC CONN
pins as possible with no
and a 0.1- F bypass
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